The present invention relates to automatic saturation control circuitry for a color television receiver in which a vertical interval reference (VIR) signal containing unique saturation information is relied upon for automatically establishing saturation correction in a displayed image.
Video information transmitted to a television receiver comprises a luminance signal portion and a chrominance signal portion. Demodulation of the chrominance signal produces color difference signals which are applied to a matrix amplifier where they are combined with the luminance signals to produce red, green, and blue color signals for application to a color picture tube.
The ratio of the chrominance signal, from which the red, green, and blue color signals for the picture tube are derived, to the luminance signal used to derive these color signals, determines the saturation or chroma gain of the receiver. As the ratio increases, the resulting picture is more saturated and as the ratio decreases the resulting picture is less saturated.
The present invention relates to control circuits of the general type set forth in U.S. Pat. No. 3,950,780 issued to Harry T. Freestone and assigned to the assignee of the present invention, in which the saturation of the television receiver is set in accordance with color information contained in the VIR signal.
The Freestone patent relies on the proposition that when the phase of the chrominance reference portion of the VIR signal is the phase of one color difference signal then the other color difference color signal in quadrature therewith is zero. Thus, if the phase of the chrominance reference portion is -(B-Y), then the R-Y color difference signal detector output should be at a null. This unique characteristic permits the use of a feedback control circuit to set the hue of the receiver by automatically adjusting the tint control until the R-Y output of the color difference detector is at a null.
In addition, the Freestone patent relies on the proposition that the fixed relationship of the amplitude of the chrominance signal during the chrominance reference portion of the VIR signal to the amplitude of the luminance signal enables the receiver processing circuitry to be adjusted so that the matrixing of the -(B-Y) chrominance signal with the Y luminance signal will provide a null, i.e., the matrix amplifier should have the same blue signal output level during the chrominance reference portion of the VIR signal that it has during black level transmission. This unique characteristic permits use of a feedback circuit to set the saturation of the receiver by automatically adjusting the relative magnitudes of the luminance and chrominance signals until the blue signal output of the matrix amplifier is at a null.
Employing this latter proposition, the Freestone patent utilizes a control loop to compare the blue signal output derived from the chrominance reference portion of the VIR signal to a black level reference signal and to adjust the saturation setting of the receiver until any difference between the two signals is zero. In a preferred embodiment, Freestone employs double interrogation of the VIR signal, once during the chrominance reference portion to obtain the blue signal and once during the black level portion to obtain the black level reference.
The present invention utilizes a different approach to that adopted by Freestone. The present invention utilizes single interrogation of the VIR signal and AC coupling of both the luminance and chrominance signals during the chrominance reference portion of the VIR signal to a matrix where the AC variations are compared to establish the control signal for setting the saturation of the receiver. Thus, no direct attempt is made to establish the null of the blue color signal. Instead, the AC variations in the luminance signal and the B-Y color difference signal are compared during VIR chrominance reference time to establish saturation control.
It is accordingly an object of the present invention to provide an automatic saturation control circuit for a color television receiver.
Another object of the present invention is to provide automatic saturation control in a color television receiver wherein single interrogation of the VIR signal is realized.
By use of a black level reference, such as obtained in a preferred embodiment by double interrogation of the VIR signal, Freestone is able to set saturation of the receiver without being subject to drift due to power supply shift or component heating or aging. Each occasion of the VIR signal provides updating of both the chrominance reference information and the black level reference so that the control loop is insensitive to drift. The control system of the present invention is also insensitive to drift by the utilization of AC coupling to compare only AC variations of the B-Y and Y signals during VIR chrominance reference time.
It is accordingly another object of the present invention to provide automatic saturation control in a color television receiver that is essentially insensitive to drift problems.